Muscle weakness is a major factor contributing to declines in functional mobility and is a strong predictor of mortality. Muscle weakness and wasting is also one of the most common pathologic clinical symptoms of environmental exposure to arsenic. However, the mechanisms and molecular pathogenesis in the etiology of arsenic-induced muscle morbidity is relatively unknown. In addition, although the contribution of arsenic exposure on stem cell function important for development has been increasingly investigated, there is an under-recognized need to know whether and how environmental toxicants like arsenic affect adult stem cell behavior and it is important to resolve how stem cell vulnerability to environmental contaminants affects the ability of otherwise healthy tissues to respond to acute injury. An enhanced understanding of the mechanisms underlying the clinical symptoms of skeletal muscle dysfunction is crucial for the design of strategies and policies to prevent or reduce injury. As a first step towards this end, the objective of this proposal is to investigate the impact and mechanisms of arsenic on pathogenic alterations of muscle function and regenerative capacity. We find that low to moderate environmental exposure to arsenic in drinking water damages skeletal muscle by disrupting muscle composition and structure, as well as injuring mitochondria and altering mitochondrial bioenergetics. In addition to myofiber pathology, we find sustained alteration of adult muscle stem cell function, including an impaired differentiation, aberrant mitochondrial bioenergetics, and increased autophagy. Interestingly, this stem cell dysfunction induced by arsenic in vivo is sustained in isolated cells cultured over serial passages, even after the arsenic stimulus has been removed. The preliminary findings support investigation of our central hypothesis that lasting mitochondrial alterations stimulated by As(III) alter myofiber and MuSC bioenergetics to impair muscle maintenance and regenerative capacity. In Specific Aim 1, we will investigate whether commonly encountered arsenic exposures induce myofiber mitochondrial dysfunction and altered bioenergetics to decrease muscle development, maintenance, and exercise capacity. In Specific Aim 1, we will investigate whether direct arsenic effects on muscle stem cell bioenergetics and function result in decreased skeletal muscle regenerative potential. Success in these aims will increase our understanding of the pathogenesis of arsenic-induced losses in muscle maintenance and clinical symptoms of weakness and will elucidate skeletal muscle microenvironmental factors controlling stem cell declines. This will ultimately lead to future mechanistic investigations designed to improve functional outcomes in patients living in arsenic-endemic areas and to create prevention strategies.